1. Field of the Invention
The present invention relates in general to a gradient coil as used in magnetic resonance tomography (MRT). The present invention relates to a method for producing a gradient coil.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear spin resonance and has been used successfully as imaging method for about 15 years in medicine and in biophysics. In this method of examination, the object is exposed to a strong, constant magnetic field. This aligns the nuclear spins of the atoms in the object, which were previously oriented irregularly. Radio-frequency waves can now excite these “ordered” nuclear spins to a specific oscillation (resonant frequency). In MRT, this oscillation generates the actual measuring signal (RF response signal) which is picked up by suitable receiving coils.
Having exact information as to the respective point of origin of the RF response signal (location information or location coding) is a precondition for the image reconstruction. This location information is obtained by additional magnetic fields (magnetic gradient fields) in relation to the static magnetic field along three spatial directions. These gradient fields are small by comparison with the main field and are produced by additional coils in the patient opening of the magnet. The overall magnetic field is different in each volumetric element owing to these gradient fields, and thus so is the resonant frequency. If a defined resonant frequency is emitted, it is thus possible to excite only those atomic nuclei that are at a location at which the magnetic field satisfies the appropriate resonance condition. It is possible by suitably changing the gradient fields for the location of such a volumetric element in which the resonance condition is satisfied to be displaced in a defined fashion, and thus to scan the desired region.
The method permits a free choice of the layer to be imaged, as a result of which it is possible to obtain tomographic images of the human body in all directions. MRT currently uses applications with high gradient performance which permit an excellent image quality with measuring times of the order of magnitude of seconds and minutes.
Continuous technical development of the components of MRT machines and the introduction of high-speed imaging sequences have created ever more fields of use for MRT in medicine. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The basic design of one of the central components of such an MRT apparatus is illustrated in FIG. 3. It shows a basic field magnet 1 (for example an axial superconducting air-coil magnet with active stray field screening) which generates a homogeneous magnetic basic field in one direction, for example, the z-direction, in the interior. The superconducting magnet 1 has, in its interior, superconducting coils which are located in liquid helium. The basic field magnet 1 is surrounded by a two-shell tank which is made from stainless steel, as a rule. The inner tank, which contains the liquid helium and serves in part also as a winding body for the magnet coils, is suspended at the outer tank, which is at room temperature, via fiberglass-reinforced plastic rods which are poor conductors of heat. A vacuum prevails between the inner and outer tanks.
The cylindrical gradient coil 2 in the interior of the basic field magnet 1 is inserted concentrically into the interior of a support tube by means of support elements 7. The support tube is delimited externally by an outer shell 8, and internally by an inner shell 9.
The gradient coil 2 has three component windings which generate respective gradient fields, proportional to the current impressed in each winding, and which are spatially perpendicular to one another. As illustrated in FIG. 4, the gradient coil 2 has an x-coil 3, a y-coil 4 and a z-coil 5, which are respectively wound around the coil core 6 and thus generate respective gradient fields, expediently in the direction of the Cartesian co-ordinates x, y and z.
The x-coil 3 and y-coil 4 are so-called saddle coils that, although overlapping one another in the end region, are generally rotated relative to one another by 90° with reference to the z-axis. The z-coil 5 constitutes a conventional Maxwell coil.
The conventional method for producing these individual gradient coils—in particular the saddle coils—is very complicated, and is summarized as follows:
To date, the coils have been produced from commercially available enamel-insulated copper wire. The wire is wound to form the coil in special winding forms (two-dimensional in the case of saddle coils), and wound onto a support. Subsequently, the latter is bent into the shape corresponding, for example, to the saddle coil, and mounted on the gradient coil 2. In the next step, the coils are connected by appropriately soldering the coil ends. Lastly, the coils are subjected to a chilled casting in order to achieve the acquired mechanical stability.
This conventional method of production is time-consuming and cost-intensive.